Histone deacetylase 6 (HDAC6) is a member of a family of amidohydrolases commonly referred as histone or lysine deacetylases (HDACs or KDACs) as they catalyze the removal of acetyl groups from the ε-amino group of lysine residues from proteins. The family includes 18 enzymes which can be divided in 3 main classes based on their sequence homology to yeast enzymes Rpd3 (Class I), Hda1 (Class II) and Sir2 (Class III). A fourth class was defined with the finding of a distinct mammalian enzyme—HDAC11 (reviewed in Yang, et al., Nature Rev. Mol. Cell Biol. 2008, 9:206-218 and in Saunders and Verdin, Oncogene 2007, 26(37):5489-5504). Biochemically, Class I (HDAC1, 2, 3, 8) and Class II (HDAC4, 5, 6, 7, 9, 10) and Class IV (HDAC11) are Zn2+-dependent enzymes, while Class III (SIRT1-7) are dependent on nicotinamide adenine dinucleotide (NAD+) for activity. Unlike all other HDACs, HDAC6 resides primarily in the cytosol. It has 2 functional catalytic domains and a carboxy-terminal Zn2+-finger ubiquitin binding domain that binds ubiquitinated misfolded proteins (Kawaguchi et al., Cell 2003, 115(6):727-738), ubiquitin (Boyaullt et al., EMBO J. 2006, 25(14): 3357-3366), as well as ubiquitin-like FAT10 modifier (Kalveram et al., J. Cell Sci. 2008, 121(24):4079-4088). Known substrates of HDAC6 include cytoskeletal proteins α-tubulin and cortactin; β-catenin which forms part of adherens junctions and anchors the actin cytoskeleton; the chaperone Hsp90; and the redox regulatory proteins peroxiredoxin (Prx) I and Prx II (reviewed in Boyault et al., Oncogene 2007, 26(37):5468-5476; Matthias et al., Cell Cycle 2008, 7(1):7-10; Li et al., J. Biol. Chem. 2008, 283(19):12686-12690; Parmigiani et al., Proc. Natl. Acad. Sci. USA 2009, 105(28):9633-9638). Thus, HDAC6 mediates a wide range of cellular functions including microtubule-dependent trafficking and signaling, membrane remodeling and chemotactic motility, involvement in control of cellular adhesion, ubiquitin level sensing, regulation of chaperone levels and activity, and responses to oxidative stress. All of these functions may be important in tumorigenesis, tumor growth and survival as well as metastasis (Simms-Waldrip et al., Mol. Genet. Metabolism 2008, 94(3):283-286; Rodriguez-Gonzalez et al., Cancer Res. 2008, 68(8):2557-2560; Kapoor, Int. J. Cancer 2009, 124:509; Lee et al., Cancer Res. 2008, 68(18):7561-7569). Recent studies have shown HDAC6 to be important in autophagy, an alternative pathway for protein degradation that compensates for deficiencies in the activity of the ubiquitin proteasome system or expression of proteins prone to form aggregates and can be activated following treatment with a proteasome inhibitor (Kawaguchi et al., Cell 2003, 115(6):727-738; Iwata et al., J. Biol. Chem. 2005, 280(48): 40282-40292; Ding et al., Am. J. Pathol. 2007, 171:513-524, Pandey et al., Nature 2007, 447(7146):860-864). Although the molecular mechanistic details are not completely understood, HDAC6 binds ubiquitinated or ubiquitin-like conjugated misfolded proteins which would otherwise induce proteotoxic stress and then serves as an adaptor protein to traffic the ubiquitinated cargo to the microtubule organizing center using the microtubule network via its known association with dynein motor protein. The resulting perinuclear aggregates, known as aggresomes, are then degraded by fusion with lysosomes in an HDAC6- and cortactin-dependent process which induces remodeling of the actin cytoskeleton proximal to aggresomes (Lee et al., EMBO J. 2010, 29:969-980). In addition, HDAC6 regulates a variety of biological processes dependent on its association with the microtubular network including cellular adhesion (Tran et al., J. Cell Sci. 2007, 120(8):1469-1479) and migration (Zhang et al., Mol. Cell. 2007, 27(2):197-213; reviewed in Valenzuela-Fernandez et al., Trends Cell. Biol. 2008, 18(6):291-297), epithelial to mesenchymal transition (Shan et al., J. Biol. Chem. 2008, 283(30):21065-21073), resistance to anoikis (Lee et al., Cancer Res. 2008, 68(18):7561-7569), epithelial growth factor-mediated Wnt signaling via β-catenin deacetylation (Li et al., J. Biol. Chem. 2008, 283(19):12686-12690) and epithelial growth factor receptor stabilization by endocytic trafficking (Lissanu Deribe et al., Sci. Signal. 2009, 2(102): ra84; Gao et al., J. Biol. Chem. 2010, 285:11219-11226); all events that promote oncogenesis and metastasis (Lee et al., Cancer Res. 2008, 68(18):7561-7569). HDAC6 activity is known to be upregulated by Aurora A kinase in cilia formation (Pugacheva et al., Cell 2007, 129(7):1351-1363) and indirectly by farnesyl transferase with which HDAC6 forms a complex with microtubules (Zhou et al., J. Biol. Chem. 2009, 284(15): 9648-9655). Also, HDAC6 is negatively regulated by tau protein (Perez et al., J. Neurochem. 2009, 109(6):1756-1766).
Diseases in which selective HDAC6 inhibition could have a potential benefit include cancer (reviewed in Simms-Waldrip et al., Mol. Genet. Metabolism 2008, 94(3):283-286 and Rodriguez-Gonzalez et al., Cancer Res. 2008, 68(8):2557-2560), specifically: multiple myeloma (Hideshima et al., Proc. Natl. Acad. Sci. USA 2005, 102(24):8567-8572); lung cancer (Kamemura et al., Biochem. Biophys. Res. Commun. 2008, 374(1):84-89); ovarian cancer (Bazzaro et al., Clin. Cancer Res. 2008, 14(22):7340-7347); breast cancer (Lee et al., Cancer Res. 2008, 68(18):7561-7569); prostate cancer (Mellado et al., Clin. Trans. Onco. 2009, 11(1):5-10); pancreatic cancer (Nawrocki et al., Cancer Res. 2006, 66(7):3773-3781); renal cancer (Cha et al., Clin. Cancer Res. 2009, 15(3):840-850); and leukemias such as acute myeloid leukemia (AML) (Fiskus et al., Blood 2008, 112(7):2896-2905) and acute lymphoblastic leukemia (ALL) (Rodriguez-Gonzalez et al., Blood 2008, 112(11): Abstract 1923).
Inhibition of HDAC6 may also have a role in cardiovascular disease, i.e. cardiovascular stress, including pressure overload, chronic ischemia, and infarction-reperfusion injury (Tannous et al., Circulation 2008, 117(24):3070-3078); bacterial infection, including those caused by uropathogenic Escherichia coli (Dhakal and Mulve, J. Biol. Chem. 2008, 284(1):446-454); neurological diseases caused by accumulation of intracellular protein aggregates such as Huntington's disease (reviewed in Kazantsev et al., Nat. Rev. Drug Disc. 2008, 7(10):854-868; see also Dompierre et al., J. Neurosci. 2007, 27(13):3571-3583; Kozikowski et al., J. Med. Chem. 2007, 50:3054-3061) or central nervous system trauma caused by tissue injury, oxidative-stress induced neuronal or axomal degeneration (Rivieccio of al., Proc. Natl. Acad. Sci. USA 2009, 106(46):19599-195604); and inflammation, including reduction of pro-inflammatory cytokine IL-1β (Carta et al., Blood 2006, 108(5):1618-1626), increased expression of the FOXP3 transcription factor, which induces immunosuppressive function of regulatory T-cells resulting in benefits in chronic diseases such as rheumatoid arthritis, psoriasis, multiple sclerosis, lupus and organ transplant rejection (reviewed in Wang et al., Nat. Rev. Drug Disc. 2009, 8(12):969-981).
Given the complex function of HDAC6, selective inhibitors could have potential utility when used alone or in combination with other chemotherapeutics such as microtubule destabilizing agents (Zhou et al., J. Biol. Chem. 2009, 284(15): 9648-9655); Hsp90 inhibitors (Rao et al., Blood 2008, 112(5)1886-1893); inhibitors of Hsp90 client proteins, including receptor tyrosine kinases such as Her-2 or VEGFR (Bhalla et al., J. Clin. Oncol. 2006, 24(18S): Abstract 1923; Park et al., Biochem. Biophys. Res. Commun. 2008, 368(2):318-322), and signaling kinases such as Bcr-Abl, Akt, mutant FLT-3, c-Raf, and MEK (Bhalla et al., J. Clin. Oncol. 2006, 24(18S): Abstract 1923; Kamemura et al., Biochem. Biophys. Res. Commun. 2008, 374(1):84-89); inhibitors of cell cycle kinases Aurora A and Aurora B (Pugacheva et al., Cell 2007, 129(7):1351-1363; Park et al., J. Mol. Med. 2008, 86(1):117-128; Cha et al., Clin. Cancer Res. 2009, 15(3):840-850); EGFR inhibitors (Lissanu Deribe et al., Sci. Signal. 2009, 2(102): ra84; Gao et al., J. Biol. Chem. 2010, 285:11219-11226) and proteasome inhibitors (Hideshima et al., Proc. Natl. Acad. Sci. USA 2005, 102(24):8567-8572) or other inhibitors of the ubiquitin proteasome system such as ubiquitin and ubiqutin-like activating (E1), conjugation (E2), ligase enzymes (E3, E4) and deubiquitinase enzymes (DUBS) as well as modulators of autophagy and protein homeostasis pathways. In addition, HDAC6 inhibitors could be combined with radiation therapy (Kim et al., Radiother. Oncol. 2009, 92(1):125-132.
Clearly, it would be beneficial to provide novel HDAC6 inhibitors that possess good therapeutic properties, especially for the treatment of proliferative diseases or disorders.